(a) Field of the Invention
This invention relates to a position pickup device for use in numerically controlled machine tools in which the moving machine part is coupled to a resolver (synchro transmitter), or the like, which is fed on the primary side with two voltages. The voltages, which are electrically shifted 90.degree. relative to each other, are generated from a clock frequency by means of a sine-cosine generator constructed from divider stages. The phase of the voltage on the secondary side of the transmitter relative to the voltage on the primary side is a measure for the value of position. More particularly it relates to an arrangement for improving accuracy in such a device.
(b) Discussion of the Prior Art
Reference is made to the arrangement described in U.S. Pat. No. 3,173,001 where voltages on the secondary side of a synchro transmitter are converted into square wave voltages derived from a clock frequency and their phase is compared with reference values in the form of square wave voltages. The control commands for the drives on the individual axes of the machine are derived from this comparison.
In computer controlled machine tools, it is necessary to form a digital signal for the actual position.
In accordance with co-pending application Ser. No. 14,431 assigned to the same assignee as the present invention, this problem is solved by generating a value proportional to the angular position of a resolver excited by a sine-cosine generator as determined at the time the output of the secondary side of the resolver crosses through zero. A dividing chain driven by a clock generator times the output of the sine-cosine generator and a second time chain, also driven by the clock generator, drives a register which counts over an interval initiated by the sine-cosine generator and ended by the zero crossing of the resolver output. The zero crossover is detected by a comparator coupled through a filter to the resolver secondary. In this way with essentially no additional costs it is possible to obtain a true position value suitable for digital processing in the computer. However, the filters and comparators used in this system, even where high grade filters are used, have a thermal phase drift which can effect the accuracy of such a device.